Soil-Bentonite Slurry Trench Cutoff Wall Final Design Report

Bucknell UniversityBucknell Digital CommonsNSF Soil-Bentonite Slurry Trench Cutoff WallDocuments NSF SB Cutoff Wall Project

2015

Soil-Bentonite Slurry Trench Cutoff Wall FinalDesign ReportUshma Manandhar

Brian Ward

Tyler Zbytek

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SOIL-BENTONITE SLURRY

TRENCH CUTOFF WALL SENIOR DESIGN REPORT

Ushma Manandhar, Brian Ward, and Tyler Zbytek

4/27/2015

SB Cutoff Wall Report Manandhar, Ward, Zbytek

1

Table of Contents Executive Summary: ..................................................................................................................................... 3

1. Introduction: .......................................................................................................................................... 3

A. Scope/Objectives: ............................................................................................................................. 4

B. Location: ........................................................................................................................................... 5

C. Background Information: .................................................................................................................. 6

2. Design Tasks: ........................................................................................................................................ 6

3. Field Tasks: ........................................................................................................................................... 8

A. Site Borings: ...................................................................................................................................... 8

4. Laboratory Tests: ...................................................................................................................................... 8

A. Moisture Content: ............................................................................................................................. 8

B. Grain Size Distribution Test: ............................................................................................................ 9

C. Atterberg Limit Test: ...................................................................................................................... 10

D. Soil Profile: ..................................................................................................................................... 11

E. Consolidation Test: ......................................................................................................................... 13

F. Triaxial Shear Stress Test: .............................................................................................................. 14

G. Triaxial Flex Wall Permeability Test: ............................................................................................. 17

5. Design Tasks: .......................................................................................................................................... 18

A. Mix Design Calculations:................................................................................................................ 18

B. Marsh Test: ..................................................................................................................................... 21

C. Slump Test: ..................................................................................................................................... 21

D. Consolidation and Permeability Test: ............................................................................................. 22

E. Consolidation Data Analysis: .......................................................................................................... 23

F. Permeability Data Analysis: ............................................................................................................ 26

H. Trench Stability and Calculations ................................................................................................... 28

I. Vertical Stress Distribution Analysis and Calculations: ................................................................. 29

6. Schedule and Challenges: ................................................................................................................... 31

7. Final Design Summary ....................................................................................................................... 32

References: .................................................................................................................................................. 35

Further Discussion: ..................................................................................................................................... 36

Discussion of the sustainability principles considered in the design project: ......................................... 36

Discussion identifying (ABET C, H) ....................................................................................................... 38

a. The realistic constraints on the project........................................................................................ 38


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b. How they were addressed in the final design .............................................................................. 38

c. The impact of your final design choices in a global, economic, environmental, or social context

38

Reflection on the design process stimulated by the following questions: ............................................... 40

a. What would you do differently if you were tasked with completing the project again? And why?

40

b. What did you learn that would influence how you will approach project work in the future? ... 40

c. What was the most difficult portion of the project? And why? How could this have been

improved? ........................................................................................................................................... 41

Discussion of at least one component of the design project that required the design team to self-learn

technical subject matter. .......................................................................................................................... 43


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Executive Summary:

Bucknell University Geotechnical Engineering faculty members have a proposal seeking

funding from the National Science Foundation (NSF) for the design, construction and long-term

research of a soil-bentonite cutoff wall at Montandon Marsh. A team of three senior civil

engineering students at Bucknell were tasked with completing the design portion of this proposal

for their senior design project. The group conducted a detailed site investigation including two

site borings and laboratory soil classification tests. Next, the group created four potential mix

designs using a composite soil mix representative of the soil stratification on site. After

consolidation and permeability testing on these soil mixes and calculation of the in-situ vertical

effective stress, the project team chose the soil mix of just the composite soil and slurry to be the

final barrier material in the wall. This mix design meets the design parameters outlined in the

NSF proposal. A construction specification was written for use by the contractor based of these

findings.

1. Introduction:

In-ground vertical barriers (i.e. cutoff walls) have been used to control the flow of

groundwater and subsurface contaminants for nearly half a century. One of the most commonly

used variations of vertical barriers is the soil-bentonite (SB) slurry trench cutoff wall that is

named for the type of final barrier materials used (SB) and the construction method (slurry

trench). While many of these cutoff walls have been built in the United States and around the

world for levee protection and in-situ geoenvironmental containment, little research has been

done on their short and long-term effectiveness. This is due to reluctance from owners,

contractors, designers and regulators to seek and report failures in completed cutoff walls. The


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long-term effectiveness of SB vertical barriers is not well understood because laboratory values

for permeability, k, obtained from field-mixed backfill specimens may be different than in-situ k

if the applied stresses in each case are different. For these reasons, a field cutoff wall built for the

main purpose of experimentation and research, in a location with long-term accessibility, would

be greatly beneficial to furthering the understanding their short and long-term performance.

A. Scope/Objectives:

This above description is the basis for a proposed NSF Grant by Prof. Evans, Prof.

Malusis and a Bucknell alumnus, now in the geotechnical contracting industry, Daniel Ruffing.

The scope this senior design project represents portions of the first two (of five) proposed project

tasks. The first of these tasks is to create a detailed site characterization along the proposed

cutoff wall alignment. This will be done by performing site borings at 50-m intervals along the

wall alignment using hollow stem drilling with an Acker Soil Scout track-mounted drilling rig

that is owned by Bucknell University. Soil cuttings will be brought back the Bucknell

Geotechnical Engineering laboratory for testing to determine soil properties such as grain size

distribution, Atterberg limits and moisture content. The results of this subsurface investigation

will be compared to previous such investigations of which the most recent was done in

November 2012. The second task that will be performed by the senior design project group is to

generate a SB mix design to be used during construction. This will require extensive laboratory

testing and experimentation and is further outlined in the design tasks section.


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B. Location:

The proposed site for the SB cutoff wall is located approximately three km east of the

Bucknell University campus as shown in Figure 1. The site for the project belongs to a

commercial sand/quarry operated by Central Builders Supply (CBS) in Montandon, PA. The site

lies adjacent to a wetland known as the Montandon Marsh. The proposed experimental cutoff

wall will be built on a section of CBS’s property that has been designated as a buffer zone

between the permitted mining area and Montandon Marsh. In recent years, CBS has allowed

Bucknell faculty and students to access the marsh area for research and other educational

activities. As the site is a wetland, the conservancy organization responsible for the protecting

this area is the Linn Conservancy. CBS and the Linn Conservancy are fully supportive of this

project.

Figure 1. (a) Index map showing the proposed site location in PA; (b) aerial photo showing

the relative proximity of the proposed cutoff wall to the Bucknell University campus; (c)

aerial photo of the portion of the quarry where the proposed cutoff wall will be located

(Evans, Malusis & Jacob, 2014).

Lewisburg

Bucknell University

Susquehanna River

Montandon

(b)

Quarry Boundary

Proposed Cutoff Wall

N

Active Mining

Mine Permit Boundary

Proposed Cutoff Wall

Buffer Zone

(a) (c)


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C. Background Information:

Prof. Evans has provided the project team with a copy of the NSF Grant proposal that

was created by Prof. Malusis, Prof. Jacobs and himself. This document has provided the project

team with much of the necessary background information for this project. One example of this is

previous site investigations that have been conducted at the proposed site. Figure 2 shows the

subsurface profile that was generated after a November 2012 site investigation by students and

faculty from Bucknell University. Other background information pertaining to the specifications

of the design of the cutoff wall was also taken from the NSF Grant proposal and is outlined in

the design tasks section of this proposal.

Figure 2. Subsurface profile along cutoff wall alignment from November 2012 site

investigation (Evans, Malusis, & Jacob, 2014).

2. Design Tasks: The design of the slurry wall will be based on data collected from subsurface

investigations along with the results of various lab tests, mix designs, and analysis. There are

various parameters for the slurry wall that must be met during the design process. The Slurry


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wall will be 300 meters in length and 0.6 meters thick. It will reach a depth of seven meters in

order to be keyed into the weathered shale aquitard. However, to ensure this depth should be

used for the entire length of the wall, borings at 50 meter intervals will be taken to ensure

homogeneity along the wall length. From these borings soil samples will be taken in order to

determine soil properties via lab testing. These borings will be completed by a hollow stem auger

drilling with an Acker Soil Scout track mounted drilling rig.

With the soil samples taken from the borings various test such as grain-size distribution,

Atterberg limits, and moisture content tests will be performed to determine the soil properties by

layer. A composite soil sample will then be mixed with bentonite in various ratios to determine

the most appropriate design for a SB backfill mixture to be used. These mixtures will be

performed by mixing approximately 5% Bentonite water slurry with different variations of

composite soil. The different mixtures will then be tested for slump and permeability. The target

Slump and permeability are 125 mm ± 12.5 mm, and k ≤ 10-8 m/s or less, respectively. The SB

mixture will best values of slump and permeability will be chosen for the design. After a

composite SB mixture has been created that meets the given parameters, specifications will be

written for use by the contractor during the construction of the cutoff wall.

To ensure the design of the slurry wall is maintained GeoSolutions, also the contractor

for this project, will be in charge of in-field testing for the slurry properties as well as the SB

backfill properties. The properties that will be tested for with the slurry are sounding, viscosity,

density, and filtrate loss. The properties that will be tested for the SB backfill are permeability,

slump, shear strength, compressibility and homogeneity.


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3. Field Tasks:

A. Site Borings:

The borings were done on two consecutive days, January 20th, and 21th. Jim Gutelius

was the driller and a Bucknell Civil and Environmental Engineering Department owned Acker

drill rig was used. The project team always had at least two team members present during the site

borings and all three project team members were able to be on site at some point during the two

days of drilling. Throughout the drilling process, boring logs for each hole were completed and

can be found in the appendix. In the field testing and visual observation was used to estimate the

classification of the soils while drilling. From these site borings auger cuttings were collected for

lab testing along with SPT (Standard Proctor Test) and Shelby tube samples. The water table was

observed to be 7.5’ below the ground surface. The first hole was drilled to a depth of 30 ft.

without hitting refusal. This hole yielded two shelby tube samples for use in laboratory testing.

The first of these samples was taken in a stiff clay layer from 16.5 feet to 18.5 feet below the

ground surface and the second sample was taken from another stiff clay layer 26.5’-28.5’ below

the ground surface. The second hole was drilled to a depth just under 20 ft., where refusal was

determined based on SPT criteria. Based on visual observation there seemed to be more sandy

material in the layers closer to the ground surface with a transition to more clayey layers at

deeper depths. The samples collected and the boring log data were then compiled to provide a

detailed site characterization.

4. Laboratory Tests:

A. Moisture Content:

The determination of the water content is a crucial aspect for the characterization of the

soil. Most natural soils that are sandy and gravelly have water contents that are approximately


9

15-20% . The natural sand grained (silty or clayey) soils have approximately (50-80%) can be

found. The soil samples from the borings were brought in the Bucknell Geotechnical

Engineering Laboratory and were immediately tested for the water content. The weight of the

empty aluminum foil pans were measured and recorded. Then they were again measured with the

soil in samples from every depth of the augur cuttings from the first boring. They were then put

in the oven for 24 hours and then the weights of the pan along with the soil were measured and

recorded again. The water content ranged from 10- 30% for the soil samples from the first

boring. Similar procedure was followed for the second boring. The moisture content was taken

for both auger cuttings and SPT samples for both the borings.

B. Grain Size Distribution Test:

Similar to the water content, grain size distribution test is essential in order to classify the

soil for the site. Sieve analysis method was performed in order to determine the grain size

distribution of the soils. This was done on all the representative soil samples that were collected

from the borings on the site from various depths. The sieves used for the analysis were sieve

numbers 4, 10, 20 ,40, 60, 100 and 200. A balance that was sensitive to 0.01 g was also used.

The mortars and rubber- tipped pestles were also used to crush the soil samples. Firstly, these

representative soil samples were put in the oven for more than 24 hours (for 2 days

approximately). The auger samples from the first boring and the SPT samples from the second

boring were taken and approximately 500g were used for the grain size distribution test. The

mass of the soil was measured and recorded. The stack of sieves was prepared and their masses

were measured and recorded before the soil was put through them. The stack was shaken for a

good 10 minutes by hand and then was placed again in the shaker for 15 minutes. After that the

mass of the soil and the pan was measured individually and recorded. If a considerable amount


10

of soil was retained on the pan, after passing through the No. 200 sieve the soil was collected

separately for the hydrometer analysis. Figure 3 shows a representative grain size distribution

graph.

Figure 3. Plot representing the grain size distribution for the soil sample retrieved between

19-20.5 feet from Boring 2.

C. Atterberg Limit Test:

This test was performed in order to find the Liquid Limit and Plastic Limit test. The first

boring had enough sample to conduct the atterberg limit test, unlike the second boring. Therefore

the atterberg limit test was conducted only on the first boring soil. For the Liquid Limit Test, the

soil samples were oven dried and the soil retrieved after passing through No. 40 sieve was used

for the test. 250g of the representative soil sample was collected and put into the evaporating

dish. A sample representative graph is shown in Figure 4. Then, water was added continuously to

make the soil like “paste”. It took more water than expected to make the soil really uniformly

moist and smooth. After that the soil sample was put in the Casagrande cup, until the maximum

depth is approximately 8mm. Then the soil sample was cut in between. The device was cranked

0

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0.010.11

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at the rate of 2 revolutions per minute and the number of counts for it to reach the closure with a

possible ½” remaining were recorded.

For the Plastic Limit test, approximately 20 grams of the soil samples were used from

each depth and water was added to them to make them smoother. The soil sample was rolled

until the diameter reached 3.0 mm and when it crumbles, take the sample and put in in the oven

and the water content was measured.

Figure 4. Plot representing the liquid Limit test for the soil sample retrieved between 5- 7.5 feet

from Boring 1.

D. Soil Profile:

After conducting the soil classification tests, the soil profile was made in order to

characterize the site. This was an important step for the project in order to know the different soil

types that exist on the site and conduct a detailed site characterization. Therefore, after

conducting the grain size distribution tests, hydrometer tests and the Atterberg limit tests, the soil

profiles in Figure 3 were determined. The soil profile from Boring 1 showed that up to the depth

0

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45

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of 8 feet below the ground, there was mostly silty sand (SM) present with the moisture content of

13.10%. Similarly for the soil profile obtained for Boring 2, it is seen that for the depth of

approximately 8 feet below the ground, it consisted of mostly silty sand (SM) as well which

consisted of moisture content of 11.10%. For both the soil profiles, the layer underneath the silty

sand, consisted of more clay particles. For the soil profile in boring 1 consisted of well graded

clayey sand (SC) , which had the moisture content of 14.98% and the soil profile for boring 2

consisted of the same soil type with a slightly different moisture content of 7.10%. This soil

profile showed that there was more clay particles deeper into the ground. This was expected

because the site location is very close to the Susquehanna River which consists of much alluvial

deposit. The soil classification tests conducted after the borings have verified that there are

primarily silt sand on site along with little gravel and cobbles that were found. This is a detailed

characterization of the site that needed to be done in order to conduct the design portion of the

wall and come up with the mix designs for the wall. However, these boring conducted are at

separate distance along the alignment of the proposed wall, therefore engineering judgment was

used in order to predict the soil types between these borings.


13

Figure 5. The two soil profiles for Boring 1 and Boring 2.

E. Consolidation Test:

Samples from the shelby tube retrieved in the first site boring at 16.5’-18.5’were used for

consolidation tests. The consolidation ring was taken and its height and mass was measured and

recorded. Three approximately 3” samples were cut from the shelby tube and was squeezed into

the consolidation ring, the remaining parts of the soil were cut out of the using a trimmer. The

mass of the specimen and the ring were measured together and recorded. Then the porous stones

were placed on the top and bottom of the ring and were lowered in the water beaker. It was

ensured that there was no movement of the ring and the porous stones were adjusted carefully to

the loading device. Load increments of 1/2 tsf, 1 tsf, 2 tsf, 4 tsf, 8 tsf and 16 tsf were used and

measurements were taken on the dial gage for determination of the time rate of consolidation.

This test showed slightly over-consolidated clay with an OCR of 3.0 and a past pressure of 2.5

tsf. Figure 6 shows the resulting consolidation curve.


14

Figure 6. Consolidation curve for clay at 17.5’ below the surface at boring 1.

F. Triaxial Shear Stress Test:

One of the very first tests performed on our soil samples was a Triaxial Isotropic

Consolidated Undrained test, CIU. This was done on the second Shelby tube sample at a depth of

26.5 ft to 28.5 ft. The Shelby tube had full recovery from the site and therefore provided three

very nice 5.75 inch samples to be tested. To run these test they were set up on a pressure

inducing board Once set up they were then consolidated each with a different effective stress,

one with 10 psi, another with 15 psi, and another with 25 psi. Due to this varying effective stress

three different effective stress circles will be provided. This distribution of different effective

stresses will be used to geometrically calculate the shear strength of the soil and determine the

cohesion, c’, of the soil. Once the samples are consolidated they are ready to be placed into the

GEOTAC machine which applies an axial stress to the sample, steadily increasing at a constant

strain rate until the strain in the reaches 15% or the soil fails in shear. These data points are

0.05

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0.25

0.30

0.1 1 10

Fin

al

Vo

id R

ati

o, e

Pressure, tsf


15

acquired automatically and are logged in a data file which is then translated into an excel file

where the data needed to perform the calculations can be found. The setup of this test is shown in

Figure 7 along with examples of how the soils tested failed in shear.

Figure 7. Set-up of trixial shear strength test (left) and sample after failure in the shear

strength test (right).

The Clay material that was found at this depth was a great sample and provided very

good results. However, one of the samples was determined to be invalid. The sample with an

effective stress of 25 psi “failed” at a lower effective stress than the sample with 15 psi effective

stress resulting in a smaller effective stress circle. It was determined to be invalid because this

shouldn’t happen. As the effective stress increases the friction forces resisting the shear will have

a higher capacity. This result could be due to a number of things, something was either wrong

with the setup of the cell or actually on the GEOTAC, there was a human error, or the GEOTAC

machine malfunctioned. Even though the 15 psi sample failed the results were still very good


16

since the 10 psi sample and 15 psi sample yielded what would be expected. The 25 psi sample in

a perfect world would have fell right of the 15 psi sample only reassuring the result is accurate.

However, as shown in Figure 8, the two samples that did work provided enough data to yield

φ=30°.

Figure 8. Mohr’s circles from triaxial shear strength tests of clay 26.5’-28.5’ from Boring 1.

Another important value extracted from these results is the c’ value, which is 1.5 psi. This

value means that the clay is slightly over consolidated which is consistent with our consolidation

rest results which yielded an OCR value of 3.0, a relatively low OCR. These results are used in

the design calculations later on in the project for trench stability and vertical stress distribution

respectively.

Two more triaxial CIU tests performed on the Shelby tube sample from 16.5’-18.5’ after

the perm cell tests were run on them in order to determine the permeability of this specific layer.

These test yielded a φ=29° and a c’=0.4545. These make sense based on the data from the first


17

test we ran and since this sample is from s shallower depth it is expected that the c’ is smaller.

Figure 9 shows these results.

Figure 9. Mohr’s circles from triaxial shear strength tests of clay 16.5’-18.5’ from Boring 1.

G. Triaxial Flex Wall Permeability Test:

The Triaxial Flex Wall Permeability test is a sophisticated way of determining the soil

hydraulic conductivity, or permeability. It is the same set up as the triaxial tests however the soils

do not have an axial stress applied until they are failed due to shear. These tests are set up in a

way which applies an effective stress of 50 psi on the soil but also applies a head pressure of 45

psi and a tail pressure of 35 psi therefore producing a 10 psi gradient within the soil sample.

After these pressures have been applied readings were taken on the cell, head, and tail

barometers. These measurements show the movement of water through the sample which will

later be used to calculate the permeability through a correlation to volume since the increments


18

on the barometers are not exactly 1 cm3. For the tests that were run, two 5.75 inch samples were

used from the shelby tube sample at 16.5’ to 18.5’ in our soil column. The results of this test are

in Figure 10. As shown, the permeability rate increases in imperviousness as time increases until

the sample reaches equilibrium where a constant permeability rate can be observed which in this

case k=10-7cm3/s. This is a very small permeability and often a target range for most cutoff walls.

This is indicative of a material that could be keyed into during construction of the wall. This

value also shows that this material will make a great cutoff wall and this type of construction is

very plausible or applicable for this sight at Montandon Marsh.

Figure 10. Permeability of Shelby tube sample at between 16.5’ and 18.5’ from Boring 1.

5. Design Tasks:

A. Mix Design Calculations:

‘Target Grain Size’ calculation was done in order to determine the target grain size

distribution of the soil composite mix that gives a good representation of the sample collected. In

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

0 5 10 15 20

k, cm

/s

Elapsed Time, days


19

order to do so, the percent passing for each sieve was determined and which was then averaged

and multiplied by the percentage of each soil layer. Finally, a percent passing was calculated for

each of the sieve by the average percent volume of the sample, which was a proper

representation of the soil sample. Similarly, the moisture content was also calculated using

percentage depth for each of the soil layer which was multiplied to the average soil moisture

content for each depth. This gave the target moisture content to be reached.

In order to reach the target mix design, certain proportions of soil from each depth were taken

and the sieve analysis was performed. After three trials of mixing the soil proportions of different

depths, the target grain size distribution was reached as shown in Table 1. The target water

content was also reached. This was checked calculating the water content using the equation

which was equal to the target water content. The grain size distribution shows that the target was

reached as shown in Figure 11.

The slurry was then made by mixing 5% Bentonite by the weight of dry mass and 95%

water. After the slurry was made, it was mixed with the soil sample that was created that had the

target grain size distribution. After that, the sample was used to take a representative sample to

take the moisture content. Then, the required amount of sample was taken in order to perform the

consolidation and permeability tests. Three other samples were made which consisted of 1% Dry

Bentonite and Slurry, 2% Dry Bentonite and Slurry and 3% Dry Bentonite and slurry by adding

1% Bentonite to the dry mass. These four samples were used to conduct the consolidation and

permeability tests.


20

Table 1. Calculation representing the target grain size distribution to create the composite

mix to represent the soil profile.

Percent Passing for each Depth(ft) Sieve Number 0-7.5 7.5-10 10-17 17-20 20-25 25-30 Composite mix

4 79.6 80.0 80.7 80.7 80.2 99.3 80.1 10 55.1 56.2 75.9 75.6 75.9 95.6 69.2 20 40.3 41.5 65.0 64.4 61.3 70.5 54.8 40 29.6 30.8 39.4 39.0 47.0 65.9 39.6 60 18.9 20.2 23.8 24.0 28.8 51.5 26.0

100 12.6 14.0 15.7 16.3 19.7 26.9 16.5 200 11.0 11.0 9.7 10.7 10.5 7.5 9.7

Percent Depth 0.3 0.1 0.2 0.1 0.2 0.1 Water content 13.6 20.0 15.0 27.9 26.6 20.1 18.5

Figure 11. The grain size distribution plot for the target composite soil mixture.

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21

B. Marsh Test:

The marsh cone was used to measure the viscosity of the slurry and the density was also

measured as shown in Figure . The test was conducted by putting the slurry in the funnel through

the screen and then stopping the slurry in the cone using a finger at the end of the funnel. The

slurry was then poured into the cup and the time was measured to reach the height of 1qt on the

cup. The slurry created by the project team had an average marsh viscosity time of 34.32 seconds

over three trials which falls in the acceptable range of 32-40 seconds specified in the NSF

Proposal (Evans, Malusis, Jacob, 2013).

Figure 12. Marsh viscosity test (left) slurry density calculation (right).

C. Slump Test:

The slump test was conducted in order for the sample to reach a certain slump design of

1- 2 inches in the miniature slump cone (Malusis et al, 2008). After preparing the slurry and

mixing it with the composite mix sample prepared, the slump test was conducted. The slump for


22

the sample was just over one inch as shown in Figure 12, which was within the design

specifications.

Figure 12. The measurement of slump of the slurry and soil sample.

D. Consolidation and Permeability Test:

After finding the target grain size distribution and preparing the composite mix of soil

with the 1% bentonite, 2% bentonite and 3% bentonite with slurry and fourth sample with just

the slurry itself, a combines permeability and consolidation test was conducted on the sample.

The sample with the target grain size distribution was weighed and initially just the slurry (95%

bentonite and 5% water) was placed and then, the sample was retrieved for the test. The moisture

content was measured for the sample as soon as it was prepared. Then, the sample was placed on

an O-ring, which was fitted with a 2.5” filter paper in the ring. The diameter was recorded to be

2.50 inches with the depth of the sample to be 1 inch. The sample was filled into the o-ring with

less air as possible and covered with another filter paper on the top. Then the saturated brown

plate was placed on top to cover the sample. The setup was cleaned and checked for any leakage

after and then was connected to the permeameter with a dial gauge. The permeameter was

assembled and all the seals were tightened. There was a pipe connecting the burette to the sample

which was used to record the difference in the water level with time. After every increment of


23

loads added to the sample, the initial time was recorded after a few hours of the addition of load.

The load increments were ¼ tsf, ½ tsf, 1 tsf, 2 tsf, 4 tsf and 8 tsf. The height of the water level in

the burette was measured and recorded along with the time, for all the loads added to the sample.

Along with the permeability, the consolidation was also measured from the dial gauge that was

connected to the sample. With every load increment the dial gauge readings were taken

immediately for certain time intervals.

The same procedure was followed for the other samples with the 1%, 2% and 3%

bentonite mixes. In order to prepare the different bentonite mixes, the respective percent of

bentonite by the mass was added to the composite mix of soil and the same procedure was

followed accordingly.

E. Consolidation Data Analysis:

After the consolidation tests were finished, the collected data was organized and

analyzed. It became evident that the amount of dry bentonite in a sample directly impacted the

results of the consolidation tests. Figure 13 shows the consolidation curves for each of the four

soil mix design samples. It can be seen that while the initial void ratio was similar for all four

samples, the void ratio decreased more drastically with increasing dry bentonite. The sample

without any dry bentonite and the sample with 1% dry bentonite followed similar curves but

there was a significant difference between them and the other two samples. The sample without

any dry bentonite followed a typical consolidation curve as it was concave down for the first part

of it and then had a near-linear recompression curve when the weight was systematically

reduced. This typical curve form was not observed in the other three samples though. Each of the

three samples with dry bentonite has a concave up first part of its curve with increasing

concavity pairing with increasing added dry bentonite. This result was not expected but can most


24

likely be explained due to the balance between the mechanical pressure supplied by the

consolidation apparatus and the swelling pressure due to the added bentonite. The sharp decrease

in void ratio is due to the swelling in the clay and not due to the mechanical pressure at low

stresses and then at higher mechanical pressures this causes the decrease in void ratio, thus

resulting in the opposite concavity of the early part of the curve. This phenomenon was likely

evident in these tests because the project team moved quickly between creating the sample mixes

and starting the consolidation tests, thus not giving the bentonite time to swell prior to the test

beginning.

Figure 13. Final void ratio as a function of pressure (tsf) for the four composite soil mix

designs.

The coefficient of compressibility, cv, is one of the important terms that was able to be

calculated based on the results of the consolidation tests. A cv value was calculated for each soil

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.1 1 10

Fin

al

Vo

id R

ati

o, e

Pressure, tsf

No Dry Bentonite

1% Dry Bentonite

2% Dry Bentonite

3% Dry Bentonite


25

sample at each loading pressure and these are shown in Figure 14. These values show how much

area of soil is compressed per unit time and it was expected that the samples with added dry

bentonite would display lower compressibility terms. Decreased compressibility is a desirable

trait of a cutoff wall because it means the wall is set much quicker and the wall properties

become more predictable. This pattern was found in the data from the consolidation tests. As the

dry bentonite increased the range of cv values decreased such that the soil-slurry mix’s curve

follows a power function regression curve while the 3% dry bentonite distribution is much closer

to linear. There is a less significant difference in the compressibility of the four mix designs at

lower pressures as there is at higher pressures which was also expected and compression is

induced by pressure.

Figure 14. Cv values (in2/sec) of the four mix designs as a function of pressure (tsf).

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0 1 2 3 4 5 6 7 8 9

Cv

10

^3

(in

2/s

ec)

Pressure (tsf)

No Dry Bentonite

1% Dry Bentonite

2% Dry Bentonite

3% Dry Bentonite


26

F. Permeability Data Analysis:

After collecting the data for the permeability test, the analysis showed that the value of k

(cm/sec) decreased with the increasing percent of bentonite. The test yielded reliable data and

showed the expected trends. Figure 15 shows the trend of permeability decreasing as the amount

of added dry bentonite increased. This was expected because an increase in the percentage of

bentonite in the soil sample will allow less water to flow through it because the bentonite will

have fewer voids due to swelling and therefore will allow less what to flow through. This trend

can be seen at regardless of what pressure the soil sample was under. Similarly, when the

pressure was increased the permeability decreased. This occurred for all the slurry and the

bentonite mixes. This was also expected because when the pressure is increased the soil becomes

more compact which makes it harder for water to flow through it.

All the mix designs satisfied the criteria of 10-6 cm/ s which was the design constraint

chosen. This value was chosen because it will make sure the wall is representative of other soil-

bentonite cutoff walls, therefore ensuring that the research conducted on the wall can be

generalized for industry use. The composite soil mix with just the slurry showed that the k value

ranged from 10-6 cm/ s to 10-7 cm/s which is a good range and close to the realistic value which

would make it easier to compare with the other cutoff walls. This range matched closely to the

design specification range which required the value of k to be less than or equal to 10-6 cm/s.

Due to this result, the soil and slurry mix without any added dry bentonite was selected as

the design mixture. Making this choice also increases the economic and time efficiency of the

wall. Dry bentonite would be a relatively expensive addition to the cost of the wall so by

choosing not to use it there is a significant drop in price. Choosing the soil and slurry mix also

means future research testing will be able to be completed faster as at almost every pressure the

soil and slurry mix had the largest permeability. Figure 15 shows that for the 3% Bentonite, it


27

takes approximately 10-8 cm/s at 4 tsf whereas it takes approximately 10-7 cm/s for the slurry

only mix. Therefore, it would take 10 times as long to run a permeability test on the 3%

bentonite mixture than it would on the soil and slurry mix.

This also confirms that the excavated soil can be used to create the final barrier material

because this is what was used to conduct these tests and the results meet the given parameters. If

the soil on site did not sufficiently meet the design constraints then soil would have had to have

been brought in from off site to mix with the excavated soil or the contractor would have had to

separate out certain grain sizes before making the backfill mix. Both of these options would have

been more expensive and more time consuming so it is beneficial that they do not need to be

considered.

Figure 15. Permeability (cm/sec) as a function of pressure (tsf) for all four soil mix designs.

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

0 1 2 3 4 5

k,

cm

/sec

Pressure, tsf

No Dry Bentonite

1% Dry Bentonite

2% Dry Bentonite

3% Dry Bentonite


28

H. Trench Stability and Calculations

Trench Stability calculations were performed in order to ensure the safety of the design

and offer specifications for the contractor. Figure 16 shows the basic layout of the trench during

construction and the variables in their respective places.

Figure 16. Visual representation of variables used to calculate trench stability (Evans and

Malusis, 2013).

These calculations included multiple variables which can be found in the Equations 1 and

2.

𝐵 =2𝑞 + 𝐻[(1 − 𝑚2)𝛾𝑚 + 𝑚2𝛾′]

𝐻(𝑛2𝛾𝑠 − 𝑚𝛾𝑤) (1)

𝐹 =2√𝐵 tan(𝜑)

𝐵 − 1 (2)

Where B is the dimensionless stability index, F is the factory of safety, m is the ratio between

height of the water table, Hw, and height of the wall, H, n is the ratio between height of the

slurry, Hs, in the trench and the height of the wall. In order for these calculations to be done a


29

few assumptions had to be made. One assumption that was made was the value of Φ. Although

this value was measured for the clay through use of the triaxial tests, we determined that it was

acceptable to assume this as the Φ for the entire column to be 30, noting that it will only increase

in strength when the soil becomes more granular. This is a very conservative assumption.

Another important assumption we made is in regards to the various unit weights that were used

in calculations. For the moist unit weight of the soil above the water table we used the SPT

Empirical relationship values where the N count was used to estimate the unit weight (Das,

1985). This was also done for the saturated unit weight of the soil below the water table. The N

count for those soils were assumed to be the same as that found in the clay layer again noting

that this is a conservative assumption since sand and gravel will only increase the unit weight.

The calculations were performed in excel so that we could iteratively arrive at a design

height of slurry in the trench. The following tables show the data from our final design height.

The calculations yielded a Factor of Safety of 1.05. It is important to note that it is typical for the

factor of safety to be below one for low density slurries, around1.05 g/cm3. This value however

is not what we used in our calculation. We assumed a higher density of 1.35 g/cm3, or 84.24 pcf,

in order to account for the mixing that will occur in the actual construction of the trench. Using

these assumptions and the data we collected we concluded that the height of the slurry must be

maintained at 29 feet or higher throughout the entire construction process.

I. Vertical Stress Distribution Analysis and Calculations:

The vertical stress that the SB backfill will experience is a critical factor in design. The

maximum stress experienced by the wall will be used in conjunction with the results from the

consolidation and falling head permeability tests to determine the design permeability and cv


30

value of the wall. It is important to note that the same assumptions used regarding the soil

properties are the same for this calculation as those in the trench stability calculations. Figure 17

shows the comparison between the natural geostatic vertical stress distribution and the actual in-

situ vertical stress distribution. Figure 17 also shows the equation used to calculate the vertical

stress at a given depth. The difference in the expected and observed stress is due to the side shear

forces resisting settlement which ultimately results in a mitigation effect on the vertical stress

experienced by the backfill.

Figure 17. State of vertical stress distribution (top) and equation for vertical effective stress

at a given depth (bottom) (Ruffing, Evans and Malusis, 2010).


31

This equation takes into account the unit weights of the soils, and shear strength as depth

increases. In Figure 18 the variables are defined and shown in the respective places on a sample

cross section. As stated above this is a vital calculation with regards to our design, providing us

with the pressure that we will use to pick out the corresponding permeability rate and cv value. In

summary due to the nature of the soil backfill the settlement experienced within the trench

creates shear forces that counteract the pure geostatic stress.

Figure 18. Factors affecting the vertical stress in a vertical cutoff wall (Ruffing, Evans and Malusis,

2010).

6. Schedule and Challenges: At the end of the 2014 Fall semester the project team created a proposed schedule of

work to be down in during the 2015 Spring semester. This schedule was updated as necessary

during the Spring semester and the latest version is shown in Table 2. Week one is designated as

starting on Wednesday January 14, 2015 and the schedule runs through the end of the 2015

Spring semester. Extra time was allotted for the tasks that the project team foresaw potential

challenges with. The project team was able to stay on this schedule without any significant

challenges throughout the course of this project. This was in large part because the field drilling


32

portion of the project was able to be completed early in the semester despite wintry weather. This

allowed the laboratory work to begin on schedule and other work to be completed when

expected. Some of the laboratory tests such as the consolidation tests were time sensitive and

time intensive which required advanced planning by the project team to make sure these tests

were progressing as scheduled.

Table 2. Proposed schedule and work breakdown.

7. Final Design Summary After completing the detailed site investigation and the mix design phases of the project

the project team completed a final design for the soil bentonite slurry trench cutoff wall. This

Task Work BreakdownPre-

Semester1 2 3 4 5 6 7 8 9 10 11 12 13 14

Schedule Field Work x

Perform Field Work Site Borings x x x

Collection of Cuttings x x x

Laboratory Testing of Boring

Samples

Grain Size Distribution

(Sieve and Hydrometer)x x x

Atterberg Limits x x x

Moisture Content x x x

Consolidation x x x x

Shear Strength x x x x

Permeability x x x x

Calculate Potential Mix Designs x

Laboratory Testing of Mix

DesignsSlump x x

Consolidation x x x x

Shear Strength x x x x

Permeability x x x x

Examine Test Results Rank Alternatives x

Choose Best Alternative x x

Write Specifications x x x

Write Report x x x x x x x x x x x

Presentation Create x x x

Practice x x

Final Presentation x

Time (Week)


33

design includes three sections: configuration, slurry mix design, and backfill mix design. This

design is prefaced by the recommendation for further site investigation along the wall alignment

as the two borings that were taken provided some different information in terms of the depth to

bedrock and thickness of certain soil layers. A more expansive site investigation could garner

further information to create a more reliable subsurface profile. The configuration design of the

wall is two feet wide and 30 feet wide. This width should be suitable to obtain strong research

results while being economical in terms of the amount of soil that is excavated and the

equipment necessary for the contractor to perform the work. This design depth will allow for the

wall to key into the thick clay layer which was found near 26.5’ below the ground surface. The

design depth may need to be adjusted based on the further site investigation as bedrock has been

found and three different depths in the three most recent borings at the site (the 30 feet design

depth is based on the first boring the project team completed).

A slurry mix of 95% water and 5% bentonite was determined to produce the required

values for viscosity and density and thus can be used for the wall. A calculation of trench

stability during construction yielded that the slurry level would need to be maintained at 29 feet.

Based on the vertical stress distribution calculation, less than 0.25 tsf is expected to be

the effective stress on the soil. This, along with the permeability results, led the project team to

select the slurry-soil mix design without added dry bentonite as the final backfill material. With

this decision made, the project team then went back to the test results to determine the design

permeability will be ~5x10-7 and based on the design cv value the expected t90 or time to 90%

consolidation is 16 days. The expected moisture content and unit weight of the backfill are

designed as 18% and 125 pcf respectively. These in-situ soil parameters will also need to be


34

reevaluated during final design and construction of the cutoff wall as they are heavily dependent

on the field conditions.

The design specifications for construction are included in the appendix.


35

References:

Das, B. M. (1985). Principles of Geotechnical Engineering . San Jose: Engineering Press.

Das, B. M. (1992). Soil Machanics Laboratory Manual. San Jose: Engineering Press.

Evans, J. C., Costa, M. J., & Cooley, B. (1994). The State-of-Stress in Soil-Bentonite Slurry

Trench Cutoff Walls. Geoenvironment.

Evans, J., & Malusis, M. (2013). Ground Improvement Engineering. In J. Evans, & M. Malusis,

Cutoff Walls.

Evans, J., Malusis, M., & Jacob, R. (2014). Soil Bentonite Slurry Trench Cutoff Wall at

Montandon Marsh. NSF Grant Proposal.

Malusis, M. A., Evans, J. C., McLane, M. H., & Woodward, N. R. (2008). A Miniature Cone for

Measuring the Slump of Soil-Bentonite Cutoff Wall Backfill. Geotechnical Testing

Journal.

Ruffing, D., Evans, J., & Malusis, M. (2010). Prediction of Earth Pressures in Soil-Bentonite

Cutoff Walls. GeoFlorida 2010: Advances in Analysis, Modeling & Design.


36

Further Discussion:

Discussion of the sustainability principles considered in the design project:

Soil Bentonite Cutoff walls are generally known to be very sustainable. The raw

materials that are used in this wall are naturally occurring soil and slurry which are sustainable

compared to other materials used in different walls such as sheet pile and tangent walls. Most of

these other walls use materials such as cement and steel which are comparatively less sustainable

materials.

The design of the wall definitely considered sustainability principles. For instance the

width and height of the wall was chosen in order to reduce excavation and still continue to

satisfy the research purposes of the wall. Excavation can be a hazard and can cause pollution

therefore, the design of this wall took serious consideration into that factor as well. The material

used in the construction of this wall pertains to the soil bentonite (slurry) and soil from the site.

Therefore, mostly local materials are utilized for the research purposes promote economic

viability. The resources used in the construction of the wall are important aspects of enriching

the economic sustainability since the project will include future job opportunities as well as

proper utilization of the resources.

The SB cut off wall serves as a vital component for the research, education and

development purposes for the students of engineering at Bucknell University. This wall will

serve to study the long term effectiveness of the cut off walls, which is well- connected to the

protection and conservation of the environment. This research will allow the study of the flow of

contaminants from one side of the wall to the other. Therefore this will help raise awareness

about environmental issues. This wall will serve towards learning more important goals of

limiting the inward seepage of water to control pollution effects which are an important aspect of


37

sustainability. The environment can benefit from this aspect hence promoting sustainability.

There will not be any degradation done to the environment while conducting the construction

process for the wall.

Because the design of this wall is for purposes of research, it is a great opportunity to

build social responsibility, awareness and enhance intergenerational equity since this wall will be

provided as a tool for any of the Bucknell campus community members to learn and advance in

the interested field of geotechnical engineering. This will help enhance the social aspect of

sustainability.


38

Discussion identifying (ABET C, H)

a. the realistic constraints on the project

b. how they were addressed in the final design

c. the impact of your final design choices in a global, economic, environmental, or

social context

This project had some realistic constraints that fall into varied categories such as

economic, environmental, and social. These constraints helped shape the way this project was

thought about and ultimately how it was completed. Some of these constraints included, the

location of the project, the pending funding or the project, and the construction of the project.

The location of this project produced quite a few constraints. The vertical cutoff wall has

been proposed to be built between an active quarry and a protected wetland. This is potentially

problematic because if the wall is built too close to the quarry then it could take away from their

potential earnings. On the other hand, if the wall is built too close to the protected wetland, then

it could disturb the balanced ecosystem that Linn Conservancy has worked so hard to create and

maintain. These constraints fall into the categories of environmental, economic, social and

potentially political. This was addressed in the final design by having the wall be built in a buffer

zone that exists between to two properties. This issue will still need to be considered during the

construction phase of the project as well though. The cutoff wall’s contractor will not be able to

store materials on the site of the quarry without Central Builder Supply’s permission and

similarly will not be able to disrupt Montandon Marsh.

As this project has been proposed for a National Science Foundation (NSF) grant, the

funding has not officially been secured yet and poses a potential economic constraint. Despite

this, the project team has worked under the assumption that the project will be funded and the

design will be used. In this way, this was less of a constraint for the team as it could be for the


39

principal investigators of the NSF grant proposal. After the project is approved and funded, the

principal investigators will be able to further the design and pay for any professionals necessary

to do this.

This project has the potential for a large impact to the geotechnical engineering triple

bottom line, in terms of economic, environmental and social impacts. Soil-bentonite walls are

more sustainable than other cutoff wall methods mostly based on the materials used during their

construction and their lack of long-term maintenance or degradation. The two other main types

of cutoff walls are sheet-pile walls and secant/tangent pile walls. Sheet-pile walls are made of

metal and therefore require an energy intensive manufacturing process. Secant/tangent pile walls

are made of concrete which therefore also requires an energy intensive manufacturing process to

create the final barrier material of the wall. Also, because both of these types of walls are made

of unnatural materials they have the potential to react and damage the ground or become

damaged and ineffective over time. Soil-bentonite walls often only require the soil on site being

excavated which eliminates the manufacturing process and therefore has economic and

environmental benefits. Soil-bentonite walls also generally require less specialized equipment

than other cutoff walls and therefore have fewer barriers to entry for contractors which can allow

for local contractors to be able to perform this work, thus having a positive social impact.


40

Reflection on the design process stimulated by the following questions:

a. What would you do differently if you were tasked with completing the project

again? And why?

There were certain segments of the project that could take more time to complete a

thorough analysis than what the semester allowed. For instance, we could have done possibly

more than two borings in order to be more accurate in our prediction for the soil profiles of the

site. In addition to that, these were other tests such as the geophysics test which included the

Seismograph test and Ground Penetrating Radar (GPR), that we wanted to conduct. However,

due to the limitation of time, we were not able to conduct those tests. This could have given

better information regarding the exact alignment of the cutoff. This could have provided us with

a more precise location of the bedrock relative to the surface of the soil which would be a large

factor when it comes to how the wall should be aligned and what subsurface component will

offer the best material to construct the wall. Similarly, some of the laboratory tests such as the

permeability, triaxial and consolidation took a little longer than predicted because of less

information about how to properly set the lab equipment. This could definitely have been

improved for the future.

b. What did you learn that would influence how you will approach project work in the

future?

This project was a great experience that influenced us in a positive way. This was a great

way to transition into the workforce after school because it provided with vigorous experience on

managing and designing various aspects of a project. This is of vital importance since it will

provide a great learning experience on how to conduct projects and manage them from the start.

The project also helped learn about time management and organization which is a very important

aspect for producing good work. Similar to this, this project also helped enabled us to conduct a


41

lot of field and laboratory work which was an enriching experience. We spent a lot of time

learning how to set up equipments and conduct tests for the samples in the lab which is of vital

importance. We also gained the ability to analyze our laboratory results which helped us produce

good design values for the design of the wall. This project was a good balance of field and lab

work overall.

Other than that, there was also a lot of teamwork which is an important aspect of working

on projects. There was efficient communication and division of work in the team which helped

us meet all our mid semester deadlines on time. The project also involved a lot of self -learning,

especially some data calculations and analysis that were needed to be done. For instance, our

team did learn a few topics of geotechnical engineering in much depth than we had done

previously in our regular courses, such as permeability, trench stability and vertical stress

distribution.

The project definitely provided an attitude of willingness to learn and not be hesitant

towards approaching a problem. It encouraged team work and approach towards problem solving

at a much deeper level. It also influenced time management skills for the better.

c. What was the most difficult portion of the project? And why? How could this have

been improved?

The field work that had to be conducted in the cold month of January definitely was

difficult. It required concentration and being able to do work effectively at the field. Jim

Gutelieus definitely was a great resource who helped us at the field site. The laboratory work for

the project was tedious and definitely required experience and perseverance. There were several

set ups for our permeability, triaxial and consolidation tests, that were required to be reset.

Sometimes, we had to set up some of our laboratory set ups more than twice. However, this was


42

overcome through guidance and advice from our advisor, but it definitely was tedious. Similarly

conducting experiments such as the grain size distribution tests for more than 10- 12 samples of

soil was tedious but was worthwhile for the completion of the project. After conducting these

tests, it was definitely difficult to analyze the results without consulting our adviser. The

theoretical values sometimes did not necessarily match the values from the laboratory results and

the graphs produced did not show the trend we had expected. So analyzing these data sets were

difficult parts of the project. Other than that, there were certain times when the project consisted

of new topics that needed more learning and time, which was difficult but an important aspect of

the course itself. The design portion of the laboratory work was also difficult. Specifically,

finding the ‘target grain size’ for our soil sample was quite time consuming. Also, the

calculations for the design portion that included trench analysis, vertical stress calculations and

permeability calculations were difficult. Time managing and being able to meet up with the team

mates on a regular basis was also not very easy considering we all had other engineering

coursework to complete.

Having mentioned that, these processes are gradual improvements and being able to

conduct fieldwork and laboratory work without much difficulty, only comes with experience. So,

this was a great learning experience of being in the lab and conducting the tests. There is no

faster way to be able to be efficient at this other than more practice and the willingness to learn.

However, learning to use the ASTM manuals for the laboratory work was useful and it did make

it easier to know the parameters that the lab tests provided. Similarly, the analysis of lab results

could have been improved by researching more literature work and books but there was

definitely the time limitation. Our adviser guided us on how to perform the analysis on our data

which took considerable amount of time but helped us achieve the design for our project.


43

Discussion of at least one component of the design project that required the design

team to self-learn technical subject matter.

Throughout this project we were learning new information regarding slurry walls and the

geotechnical field as a whole. We performed lab tests that we were briefly exposed to us at new

levels of depth and accuracy, running multiple tests at a time while analyzing the tests that have

just finished. However, the design portion of this project required us to learn the most. There

were two calculations that we had to learn ourselves before we could complete them. These

calculations were the Trench Stability Calculations and the Vertical Stress Distribution

Calculations. Both of these calculations required us to research the subject matter and learn the

concepts well enough to apply them to our design.

The Trench Stability Calculations are a very specific calculation for slurry wall

construction that was not covered in the classes offered at Bucknell. This is because these

calculations are again very specific and not very useful for anything other than slurry wall

construction. This was very interesting process where we were able to take what we have learned

in the past within our classes and then expand on that knowledge with the trench stability

calculations. This was a very encouraging situation where we could use our prior knowledge and

figure out what was happening in this new situation. In order to acquire this information we were

given a chapter out of a book that our advisor Professor Evans is writing at the moment. We

primarily used this chapter to learn from and then apply it to our design project where we

actually performed the calculation to obtain a design value. The value was then designated as a

parameter that will be used during construction.

Similarly, the Vertical Stress Distribution Calculations were something that was not

taught specifically to us in our time here at Bucknell. These calculations however, built off of our


44

prior knowledge more so than the Trench Stability Calculations. With these calculations we had

to calculate the stress distribution along the height of the wall. It starts out as a simple effective

stress problem but adds a new element of difficulty when the slurry dynamics are added in. It

goes from a purely static problem to a combination of static and dynamic. This involved an

added shear force along the edges of the soil bentonite backfill which settles after it is dumped

into the trench. This mitigates the downward vertical stress on the backfill. Once, we were done

performing this series of calculations we came to a maximum vertical stress of 350 psf which is

less than 0.25 tsf. This is important because this is the value we used to determine our design

permeability and design cv value.

Both of these calculations were vital with respect to our project. Without teaching

ourselves these calculations and learning more about the subjects we wouldn’t have had a final

design for our project.